A varve is an annual layer of sediment or sedimentary rock.I guess one can think of a "varve" as the geologist's "tree ring" when looking at sediment. I don't know if there is a scientific term for "tree ring" but the study of tree rings is dendrochronology.
But I digress.
One of the ten "page tabs" at the top of the blog: geology.
I haven't added anything to that page in a long, long time.
This has nothing to do with the Bakken, so I will add it as a "note to the granddaughters."
Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock that are almost always of Precambrian age.
A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shales and cherts, often red in color, of similar thickness, and containing microbands (sub-millimeter) of iron oxides.
Some of the oldest known rock formations (having formed ca. 3,700 million years ago), are associated with banded iron formations.
Banded iron formations account for more than 60% of global iron reserves, and can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.As of 2015, the reason for the alternating stripes has not been explained but some suggest isotope signatures in the striped bands suggest a bacterial connection.